The production process of methanol utilises a reformer stage. In the reformer stage, a natural gas and water mixture moves through heated tubes. Each tube contains nickel oxide (NiO) catalyst. This allows an endothermic reforming reaction to occur.
This process is also known as steam reforming (SR), sometimes referred to as steam methane reforming (SMR).
An external source of hot gas is used to heat tubes (reformer tubes) in which the catalytic reaction takes place. This reaction converts steam and lighter hydrocarbons such as methane into hydrogen and carbon oxides (syngas). The typical product of this process includes a mixture of H2+CO+CO2 (+H2O).
Reformer tubes are hollow tubes usually vertically suspended in a plurality of rows within a reactor (furnace enclosure). The furnace may be 15 m tall and 25 m square for example, housing some 700 tubes each around 12-14 m tall. The reformer tubes in a furnace are typically suspended by springs (or counterweights) off hangers that are located above the tubes.
Process operation temperatures see the tubes subjected to temperatures in the range of 900 and 950 degrees Celsius. The approximate optimum temperature for process efficiency is 930° C. A temperature lower than this will result in significant methane not being converted (methane slip) which may affect the efficiency of the plant operation. Conversely, temperatures higher than 930° C. will result in increased creep and reduced tube lifespan. Going from 930° to 950°, the tube life span is reduced by about half.
The lifespan of a tube is determined by their slow expansion at high temperatures (creep). Tubes creep both axially and diametrically. It is diametric creep that has the predominant impact on lifespan of the tubes. Operating the reformer too hot will shorten the life of the tubes and premature and unexpected failures can hence be costly. A leak in one tube can cause damage to the surrounding tubes. Currently each tube can cost around US$20,000. Further, replacing failed tubes requires a full plant shut down, potentially costing millions of dollars of lost production.
A common method for preventing SMR overheats is to measure energy in and energy out of the furnace to ensure excessive tube temperatures are not possible. Typically, overall temperature control in the furnace is achieved by regulating fuel gas pressure over all of the burners. Individual tube temperatures are controlled by fuel gas flow valves at each burner (via trimming). Accordingly, the trimming one burner may reduce the temperature in adjacent tubes, but that may then result in a peak of temperature elsewhere in the furnace.
This global approach does not always protect individual or small groups of tubes that can be overheated through operator error or equipment malfunction. This method of overheat protection is hence not fully effective, and burnouts of the reformer can, and do, sometimes occur. There are numerous reported failures using this method.
Measurement of the temperature of individual tubes is typically achieved by sight ports through the furnace wall. The sight ports are opened or able to be opened to allow infra red instrument access to determine and measure the temperature of tubes. However, opening the sight ports (without a glass window) allows cold air into the furnace and/or hot air out, and can cause a temperature change in tubes near the sight ports. Further, the accuracy of such measurements is low and the instruments may be reading a temperature variation of up to ±20°. Further still, such a manual approach to temperature measurement is very time-consuming (for example it may take between 40 minutes to an hour for an operator to make their way around the reformer measuring temperatures). As a result, the frequency of measurement is very low and may only occur at a few times in a 24 hour period.
In addition the sight ports do not allow for visibility of all tubes to be achieved because some tubes are obscured by the thickness of the furnace refractory lining. Therefore some tubes (particularly those around the perimeter) may not get monitored, as accurately, or at all. It has also been found that this type of temperature measurement is potentially quite variable between different operators, further affecting the accuracy and reliability of temperature data.
When adverse tube temperatures are detected by the infrared instrument, the tube temperature needs to be adjusted. Temperature control of the whole furnace is achieved by fuel gas pressure over all of the burners. Temperature of individual tubes can be controlled (trimmed) by dedicated gas flow valves to appropriate adjacent burners. A person who has observed, using the infra red instrument, one tube being of a high temperature may for example turn down the dedicated valve at an adjacent burner to reduce a tube's temperature. This process may be iterative and ongoing across all tubes in the furnace. This may in part be because adjusting a change in temperature of one tube may have an adverse effect on the temperature of another tube in the furnace.
The operations management preferably control the burners in an effort to maintain a relatively even temperature throughout the entire reformer by trimming. A well trimmed reformer will generally result in the highest efficiency of the reforming process.
Creep affects the life span of a tube. A typical lifespan of a tube is approximately 11 years. Creep is currently measured each time a plant is shut down. This may be roughly every 4 years. When the plant has been shut down, a device such as that shown in US2005/0237519 can measure the inside diameter of each tube along its length. This data can be compared to the tube when new. Where the degree of measured creep has exceeded a certain predetermined limit, a decision can be made to discard the tube and replace it with a new tube because of the statistical knowledge that the old tube is likely to fail in the next four year cycle.
However these data only become available when the plant is shut down at which point it is too late to order new tubes if there are insufficient spare tubes. The temperature data derived from individual tube growth measurements can be used to calculate the tube life consumed and hence allow sufficient tubes to be held for a planned plant shut down.